1. Field of the Invention
The present invention relates to a fixing apparatus, which is built into an image forming apparatus such as copy machine and printer, and fixes a toner (developer) image on a paper sheet.
2. Description of the Related Art
Recently, induction heating has been practically used as heat source of a fixing apparatus used for an electrophotography system. According to induction heating, high-frequency current is carried to coils so that a high-frequency magnetic field can be generated by the coils. A heat roller generates an eddy current using the high-frequency magnetic field, and thereafter, self-heats using joule heat by the eddy current.
The fixing apparatus employing the foregoing induction heating has a thermostat as a thermally sensitive element for sensing the temperature of the heat roller. The thermostat operates to cut off the input current to the fixing apparatus if the temperature of the heat roller rises abnormally. Induction heating is stopped according to the cut-off, thereby preventing the temperature of the heat roller from rising abnormally.
However, in order to cut off the input current to the fixing apparatus, a thermostat having a large rated current (e.g., 10 A to 15 A) must be used. The large rated current thermostat has a large heat capacity, and is late to response speed. For this reason, a time lag is generated until the thermostat operates after the temperature of the heat roller rises abnormally. This is a factor of giving thermal influence to the heat roller and its peripheral sections.
On the other hand, JPN. PAT. APLLN. KOKAI Publication No. 9-197854 discloses the following technique. According to the technique, there is provided a thermistor, which senses the temperature of the heat roller, and a relay, which operates when the temperature sensed by the thermistor rises abnormally. The contact of the relay is inserted and connected to the conducting path to coils. More specifically, if the temperature of the heat roller rises abnormally, the relay operates to open its contacts, and thereby, conduction to the coils is cut off, so that induction heating can be stopped. However, in this case, a large current of about 60 A flows through the coils, and thus, a high voltage of about 900 V is applied to the coils. In order to cut off the foregoing large current and high voltage, a large type relay must be used. However, the large type relay for cutting off the large current and high voltage is expensive; therefore, it is not suitable for practical use.
JPN. PAT. APLLN. KOKAI Publication No. 2002-236429 discloses the following technique. According to the technique, there is provided a thermostat, which senses the temperature of the heat roller. The thermostat operates to cut off the supply of operating voltage to the drive means of a switching element if the temperature of the heat roller rises abnormally. The switching element is used for supplying a high-frequency current to coils. However, in this case, a special IC must be used as the drive means of the switching element. The special IC is expensive; therefore, it is not suitable for practical use.
JPN. PAT. APLLN. KOKAI Publication No. 8-339134 discloses the following technique. According to the technique, there is provided a thermistor, which senses the temperature of a fixing roller. In the thermistor, the resistance value varies greatly if the temperature of the fixing roller rises abnormally. When the resistance variation occurs, conduction to a fuser is cut off according to control using the IC. Induction heating is stopped by the cut-off of the conduction to prevent the temperature of the fixing roller from rising abnormally. However, control using the IC is made between temperature sensing by a thermistor and cut-off of conduction to the fuser. For this reason, if failure occurs in the control, the conduction to the fuser is not cut off. As a result, it is impossible to prevent the temperature of the fixing roller from rising abnormally.
In addition, JPN. PAT. APLLN. KOKAI Publication No. 8-339134 discloses the following structure. There is provided a relay, which operates when the temperature of the heat roller rises abnormally, and the contact of the relay is inserted and connected to the conducting path to coils. However, according to the structure, there is a problem that the large type relay for cutting off a large current and high voltage must be used like the foregoing Publication No. 8-339134.